During the production of steel in a converter furnace, impurities referred to as "slag", float atop the molten metal. It is desirable to remove the molten metal from the furnace separately from as much of the slag as possible to minimize the amount of impurities within the metal. One conventional way of achieving that result is to tilt the furnace while plugging a tap hole of the furnace so as to block the exit of slag, and then unplugging the tap hole after at least most of the slag has passed thereover, whereby molten metal will be poured from the tap hole while the slag remains in the furnace.
It is desirable that the plug create an effective seal with the surface of the tap hole in order to minimize the leakage of slag past the plug. Also, it is desirable to install the plug deeply into the tap hole in order to minimize the amount of slag which can enter the tap hole. A plug must overcome certain formidable obstacles in order to achieve those goals.
One obstacle relates to the high temperatures present within the tap hole, e.g., temperatures which may exceed 3000.degree. F., which restrict the types of materials which can be employed when forming the plug.
Another obstacle involves a non-uniform cross-sectional shape of the surface of the tap hole. That is, due to an uneven wear of that surface during the pouring of molten metal and/or a depositing of metal or slag on the surface, the surface of a well used tap hole can become highly irregular in cross-sectional size and shape. As a result, gaps tend to be formed between the plug and the tap hole surface, resulting in an appreciable leakage of slag during a tapping sequence.
Various techniques for plugging the tap hole have been proposed, see for example U.S. Pat. Nos. 3,124,854; 3,398,945; 3,540,627; 4,030,709; 4,471,950; and 4,826,139.
In U.S. Pat. No. 4,471,950 a blunt-nosed plug is formed of a ceramic material. A wedge disposed inside the plug can be pushed forwardly by a manual rod to expand the plug into engagement with the surface of the tap hole. It may be difficult to maneuver plugs of this type such that the plug is pushed deeply into the tap hole before being expanded. Furthermore, the plug provides no projection for the outer surface of the ceramic material. Any rubbing of that surface against the tap hole could result in damage to the ceramic material. In addition, the wedge is relatively short in length and thus may not impose sufficiently high forces to the rear end of the plug for effectively resisting the leakage of slag therepast.
In U.S. Pat. No. 4,826,139, a concrete plug is disclosed which can be installed deeply into the tap hole. This is made possible because the plug diameter is smaller than the diameter of the tap hole, i.e., a loose-fit exists. Bendable metal strips disposed behind the plug serve to engage the surface of the tap hole and resist outward movement of the plug until the metal strips have been softened by the heat of the molten metal during a tap sequence. It would be desirable, however, to create a better fit between the plug and the surface of the tap hole for more completely blocking the outflow of slag.
The remaining ones of the above-listed patents disclose pushing a frusto-conically shaped plug of steel or masonry into the tap hole. The cone is inserted at the end of a pole until coming to rest on the lip of the tap hole. Often, as the result of slag or steel build-up on the tap hole lip, there may be insufficient room in which to wedge the plug in place. Also, since the lip of the tap hole tends to wear and thus become enlarged after prolonged usage, it is usually necessary for the user to stock more than one plug size.
It has also been proposed to form a plug body of a special composition which, when heated to the high temperatures present in the tap hole, tends to expand into tighter contact with the tap hole surface. However, such plugs are relatively expensive.